Neem seed oil: a potent nitrification inhibitor to control nitrate leaching after incorporation of crop residues

The effect of neem seed oil and neem leaf extract as organic nitrification inhibitors (NIs) on the accumulation of NH4+ and NO3-, and nitrification inhibition after incorporation of crop residue was investigated in an incubation experiment. Dicyandiamide (DCD) applications of 15 and 30 kg active ingredient ha(-1) were used as low and high doses of a synthetic NI. Soil samples were amended with 21 g kg(-1) cauliflower leaves and treated with NIs at a rate of 30 kg ha(-1) of neem seed oil, 60 kg ha(-1) of neem leaf extract, 15 kg ha(-1) of DCD, and 30 kg ha(-1) DCD. Samples were incubated at temperatures corresponding to field temperatures during fall and winter in Flanders, Belgium. Neem seed oil increased NH4+ accumulation by 8.9 mg kg(-1) and decreased NO3- by 13.5 mg kg(-1) within a month. High and low doses of DCD increased NH4+ accumulation by 48.2 and 1.6 mg kg(-1), respectively. Nitrification was inhibited by 29% for 30 days by the low dose of DCD, 58% in 30 days by neem seed oil, and 42% in 45 days by the high dose of DCD. Nitrification was not inhibited by the neem leaf extract.